The present invention relates to a semiconductor radiation detection apparatus such as radiation monitors, individual exposure dose meters or survey meters to be used in radiation handling facilities such as nuclear power stations or reprocessing facilities.
A prior art semiconductor radiation detector is described in Japanese Patent Publication No. 56-129380 (1984). According to this prior art, a multi-layer semiconductor radiation detection element or device is provided by growing (through epitaxial crystal growth) or bonding a low specific resistance (or lowohmic) layer on a high specific resistance (or high-ohmic) semiconductor layer, and further growing or bonding another high specific resistance layer thereupon. Output electrodes are formed in contact with each of high specific resistance layers and a common electrode in the low specific resistance layer in the semiconductor radiation detection element, with a lead wire attached to each electrode, constituting a semiconductor radiation detector. By applying respective reverse bias voltages between the common electrode and each of the two output electrodes in the semiconductor radiation detector, depletion regions are formed in both of the high specific resistance semiconductor substrates. When incident radiation enters these depletion regions, a pulse signal is generated on an associated output electrode, thus effecting detection of incident radiation. When both .alpha. and .beta. rays enter and traverse the semiconductor detector, the respective components of .alpha. and .beta. rays can be distinguished and measured quantitatively separately from each other due to the difference in the penetration power. That is, while the .beta. rays can penetrate through both depletion regions, .alpha. rays do not penetrate to the second depletion region, and are therefore detected only in the first region.
In the following description, the semiconductor radiation detection apparatus refers to a semiconductor radiation detector provided with reverse bias voltage applying means, and signal processing means for processing signals from the output electrodes.
The utility of the above-mentioned prior art involving lamination or bonding processes or the like in forming multilayered semiconductor radiation detection elements is limited because it cannot be further improved in sensitivity for several reasons.
Firstly, the bonded portion or the low specific resistance layer becomes a radiation insensitivity band or dead band in which energy of .beta. rays having a large penetration factor is lost, thereby lowering detection sensitivity for .beta. rays.
Second, in the case of, for example, a silicon semiconductor, the specific resistance factor of a high specific resistance body formed by lamination is limited, at most, to several tens of Ohm centimeters (.OMEGA..cm). Assuming an applied reverse bias of 100 V, the thickness of a depletion region to be formed is 16 .mu.m at most according to equation (1) to be explained later. The necessary thickness of the depletion region, on the other hand, depends on the energy of the radiation, and when the energy of .alpha. rays is assumed to be 8 MeV, the necessary thickness becomes 60 .mu.m; thus, a maximum thickness of 16 .mu.m cannot suffice as a necessary detection sensitivity. Although it is possible to obtain a necessary depletion region by increasing the applied voltage, this requires as much as 4000 V, hence diminishing the primary advantages of low voltage operation unique to semiconductor detectors.
Third, as described previously, according to the lamination method, it is difficult to form a high specific resistance body having a specific resistance exceeding several tens of .OMEGA..cm. Because low specific resistance bodies contain many impurities and defects, substantial currents are generated in the semiconductor materials, and a leakage current per unit area in the detection element cannot be reduced. Generally higher sensitivity can be realized with larger area. According to the prior art, however, when the area is increased, the degree of the leakage current becomes far greater, thus failing to achieve the desired high sensitivity. According to the prior art, the maximum diameter practically obtainable is 5cm.
Fourth, as the prior art constructions are not capable of adapting to arbitrary energy levels, for instance, of .alpha. rays, a desired high sensitivity cannot be realized. This is because the necessary thickness of the depletion region depends upon the energy levels of radiation rays as hereinabove described, and needs to be made greater with an increased level of energy. When the thickness is greater than necessary, however, a portion of the depletion region other than required is likely to detect radiation rays other than the objects to be measured, thus becoming a noise source, and failing to provide a high sensitivity. Thus, it is necessary to be able to control the thickness of the depletion region according to the energy level of an object to be measured. According to the prior art devices, however, the thickness of the depletion layer is fixed, and is unable to satisfy the above requirements. This applies likewise where a pair or depletion regions are formed in order to discriminate at least two kinds of radiation rays, for example, such as .alpha. rays and .beta. rays as above.
A second problem associated with a laminated structure made by bonding of a semiconductor radiation detection element is that it has a low heat resistance capability and reliability due to the differences in heat expansion coefficients between the respective layers.
A third problem is that according to the lamination method, a large number of fabrication steps are required; a process of laminating a low specific resistance film on a high specific resistance film; and another lamination of a high specific resistance film thereupon, and the like.